This invention relates generally to an improved composition for producing porous ethylene vinyl acetate (EVA) coatings.
Medical devices often are sterilized by exposing a package of devices to ethylene oxide vapor. The package is enclosed in a polymer-coated paper or nonwoven material which maintains package integrity during its shelf life, keeping out bacteria and other solid contaminants. Substrates used for this purpose include spun bonded polyolefin materials and special grade medical papers. Porosity is a critical property of the polymer coating, whose gas transmission rate must be sufficient to allow effective penetration by the ethylene oxide vapor during the sterilization process. Often, EVA coatings are used for this purpose, and porosity in the coating is created by incorporating particles of solid insoluble resins, which create voids in the dried coating. Variations in particle size of the resins, drying conditions, tackifiers, and other factors can make it difficult to achieve coatings having consistent porosity through the use of solid insoluble resins. For example, U.S. Pat. No. 6,764,566 discloses nonwoven materials with a porous coating made through incorporation of solid insoluble resins. However, this reference does not disclose a method for producing consistent porosity without the use of these materials.
The problem addressed by this invention is to find an alternative composition for producing porous EVA coatings without using solid insoluble resins.